This invention relates to audio signal test and measurement apparatuses and methods, and particularly to an audio signal test apparatus and method that permits the measurement of the signal characteristics of the ac mains to which it is connected.
In the measurement of audio signals produced or applied to audio frequency signal processing equipment the signal levels to be measured may cover a wide range, for example, from a few microvolts to one hundred volts. However, most often the signal level is near the low end of that range. Consequently, most audio signal measuring instruments have a high gain input circuit to amplify low level signals to a useful level.
Often, problems in the operation of audio signal processing equipment arise as a consequence of the electrical characteristics of the ac mains supplying that equipment being other than what it should be. (The term "ac mains" is used herein to refer to the ac power source.) Ordinarily, the ac mains in the United States is supposed to provide 120 volts rms, at 60 Hz with very low distortion. In Europe the ac mains voltage is ordinarily 220, 230 or 240 volts rms, depending on the country, and the frequency is ordinarily supposed to be 50 Hz. Audio signal processing equipment is often designed to allow for some variation in the electrical characteristics of the ac mains, for example, by being able to operate normally at 110 volts instead of 120 volts, but abnormal electrical characteristics can still cause problems.
When the ac mains voltage is below normal, modern switching power supplies may quit working, and older power supplies may not supply adequate voltage or current. Distortion in the ac mains can cause over heating of a power supply and also lead to failure of the supply to provide adequate power. While the frequency of most ac mains is generally well regulated, where a significant variation in frequency occurs, a power supply may malfunction. All of these events can lead to malfunctions of audio signal processing equipment.
It would therefore be desirable to include within the same audio frequency measurement instrument that is used to measure the performance of audio signal processing equipment a means for measuring the electrical characteristics of the ac mains which supplies power to that signal processing equipment. However, conventional wisdom militates against measuring the electrical characteristics of the ac mains with the same test instrument used to measure audio signals, because of the need to keep high level 60 or 50 Hz signals away from the sensitive input circuits of such test instruments.